1. Field of the Invention
The present invention relates to a sintered synchronizing ring of the type used in gear boxes.
2. Discussion of the Related Art
A conventional gear box comprises an input shaft and an output shaft on which are distributed gears. Each gear supported by the input shaft generally permanently is in mesh with a gear supported by the output shaft. When no gear is engaged, that is, at the dead point, one of the gears, called the idle gear, of each pair of meshed gears freely rotates with respect to the associated shaft.
The selection of a gear ratio consists of making one of the idle gears solid with the associated shaft. This is done especially by means of a sliding gear rotated by the shaft supporting the idle gear to be selected and comprising dogs capable of meshing with dogs of the idle gear to make it solid with the shaft. For the meshing of the sliding gear dogs with the idle gear dogs to occur flexibly and smoothly, the rotation speeds of the gear and of the shaft must be equalized. A synchronizing ring capable of cooperating with the idle gear to progressively bring by friction the idle gear to the same rotation speed as the shaft rotation speed before the dogs of the sliding gear mesh with the dogs of the idle gear is thus provided.
FIGS. 1A and 1B respectively are a front view and an enlarged partial cross-section view of a conventional synchronizing ring. The synchronizing ring comprises ramps 10 distributed along its periphery on which the pushing force of the sliding gear (not shown) applies during the speed equalization phase. As visible in FIG. 1B, ramps 10 only extend along a portion of the ring height and flush an end thereof. Further, the ring comprises a thinned down peripheral area 12 which connects to ramps 10.
The synchronizing ring is provided with a tapered central bore 14, the largest diameter being on the side of ramps 10, as visible in FIG. 1B. The tapered bore is intended to cooperate with a tapered end of the idle gear by axial translation of the ring to ensure fast synchronization of the ring and of the gear by friction until the speeds of the tapered portions are equalized.
It is known to use brass, or a brass-based alloy, to form the synchronizing ring. Indeed, brass provides a proper coefficient of friction between the synchronizing ring and the idle gear, especially at the end of the synchronization step.
The components of a gear box are generally lubricated to avoid wearing and jamming. However, the lubrication of the tapered parts of the synchronizing ring and of the gear are incompatible with the desire to obtain a fast synchronization of the two components with respect to each other.
A conventional solution used to improve the synchronization of the synchronizing ring and of the gear consists of providing a threading at the level of tapered bore 14 of the synchronizing ring. Such a threading enables efficient evacuation of the lubrication oil when the synchronizing ring and the tapered end of the gear are brought close to each other and enables faster synchronization of the ring with respect to the idle gear. It is further also known to complete the threading with axial grooves distributed at the level of the tapered bore.
A disadvantage of such a synchronizing ring is that it requires a relatively complex manufacturing process. Indeed, the threading, which provides many undercut surfaces, cannot be obtained by a single casting or sintering operation and must thus be performed by an additional machining step, which increases the ring manufacturing cost. It is thus desired to define a synchronizing ring having a specific shape enabling its forming in a single casting or sintering operation.
Another disadvantage of the previously-described synchronizing ring made of brass or of a brass-based alloy is that a significant wearing of the synchronizing ring can be observed at the level of the dogs, which tends to reduce the ring lifetime. This is essentially due to the mechanical resistance properties of the used brass or brass alloy. It is thus desirable to be able to use a material different from brass, having a lower production cost than brass, and having improved mechanical resistance properties to form the synchronizing ring. It would be desirable to be able to use steel to form the synchronizing ring, which generally has a lower production cost than the production cost of brass and which has improved mechanical resistance properties.
To form a synchronizing ring in a single casting or sintering operation, one possibility is, as described in European patent application EP 0965769 filed by the applicant, to provide axial teeth 16 regularly distributed over the surface of bore 14, as shown in FIGS. 1B and 1C. Indeed, axial teeth 16 offer no undercut surface and can thus be obtained by sintering or casting with no additional machining operation. To obtain adequate friction properties especially enabling use of a steel to form the synchronizing ring, sharp teeth 16 arranged along the axial direction of the ring, which prevent any jamming phenomenon between the ring and the tapered end of the gear, are provided.
FIG. 2 shows curves of the variation of friction coefficient μ between the synchronizing ring and the tapered end of the gear in a synchronization step of duration ΔT. The curve in dotted lines 20 shows the desired variation of the friction coefficient in a synchronization step and approximately corresponds to the variation of the friction coefficient for a brass synchronizing ring comprising a central threaded tapered bore and a steel tapered end. For such a curve, friction coefficient μ increases rapidly towards an optimal friction coefficient μOPT and stabilizes at this value. The optimal friction coefficient corresponds to a compromise between a friction coefficient sufficiently high to enable fast synchronization and a coefficient low enough to avoid any jamming.
The variation of the friction coefficient for the synchronizing ring shown in FIGS. 1A to 1C, that is, for a steel synchronizing ring having its central bore 14 comprising sharp axial teeth 16, is shown by curve 22 in full line. It can be acknowledged that the increase of the friction coefficient up to optimal coefficient μOPT is slower than for curve 20 and that, at the end of the synchronization, the friction coefficient tends to exceed optimal coefficient μOPT. A phenomenon substantially opposite to that which can be observed with a steel synchronizing ring with a threaded internal bore for which the friction coefficient tends to keep a value smaller than optimal coefficient μOPT is thus obtained at the end of the synchronization.
Regarding curve 22, the delay taken by the friction coefficient to reach optimal coefficient μOPT translates as a less efficient synchronization of the ring and of the gear at the beginning of the synchronization, which is not desirable given the short duration of the synchronization step. Further, the value reached by the friction coefficient at the end of the synchronization step corresponds to the static friction coefficient, or adherence coefficient, between the synchronizing ring and the tapered end of the gear, and is thus representative of the efforts which will have to be subsequently made to separate the synchronizing ring from the tapered end of the gear. It is thus not desirable for such a value to be too high.